1. Field of the Invention
The present invention relates to an image heating apparatus that can be used as a heating and fixing apparatus (fixing unit) mounted to an image forming apparatus, such as an electrophotographic copying machine or an electrophotographic printer, and an endless belt used for an image heating apparatus.
2. Description of the Related Art
A film-heating-type fixing device has been known for use in a heating and fixing apparatus (fixing unit) that is mounted to an image forming apparatus, such as an electrophotographic copying machine or an electrophotographic printer. Japanese Patent Application Laid-Open No. H04-044075 discloses this type of heating and fixing apparatus. The heating and fixing apparatus includes a heater that has an electric heating resistor layer formed on a ceramic substrate, a fixing film (endless belt) that is moved while contacting the heater, and a pressure roller that contacts the fixing film to form a nip portion. A recording material having a non-fixed toner image carried thereon is heated while being pinched and conveyed in the nip portion of the heating and fixing apparatus. Then, the toner image formed on the recording material is fixed to the recording material by heat. This type of heating and fixing apparatus has an advantage in that, after the supply of power to the heater starts, the temperature of the heater increases up to a toner image fixable temperature in a short time. Therefore, a printer provided with the heating and fixing apparatus has a short first print out time (FPOT) after a print instruction is input. In addition, this type of heating and fixing apparatus has an advantage in that power consumption is small while it waits for the input of a print instruction.
The heating and fixing apparatus of the film heating type needs to rapidly heat the non-fixed toner image formed on the recording material to fix it to the recording material, in order to correspond to an increase in the process speed of a copying machine or a printer.
As described in Japanese Patent Application Laid-Open Nos. H10-319753 and 2001-225134, a thick sleeve that is made of a metal material having high thermal conductivity instead of a heat-resistant resin is used as a base layer of a fixing film to improve the heat transfer efficiency of a recording material.
In the heating and fixing apparatus of the film heating type, in some cases, while the fixing film is being rotated, deviation occurs in the fixing film in the longitudinal direction thereof. It is difficult to control the deviation force. In particular, when there is a large difference in parallelism between the pressure roller and the fixing film or when there is a large difference in pressing force between both ends of the fixing film and the pressure roller in the longitudinal direction, strong deviation force is applied to the fixing film. Then, strong stress is applied to the end of the film from which the deviation force is generated. In this case, the end of the fixing film is likely to be damaged.
Japanese Patent Application Laid-Open No. 2002-323821 discloses a structure in which a film end regulating surface of a fixing flange regulates the movement of the end of a fixing film to prevent the deviation of the fixing film.
In the heating and fixing apparatus of a film heating type, when a thick metal sleeve is used as the base layer of the fixing film, the following problems are likely to arise.
In order to achieve a heating and fixing apparatus capable of corresponding to an increase in the process speed of an image forming apparatus, generally, a pressing unit that applies pressure to a nip portion is reinforced to ensure a fixing property. When the pressing unit is reinforced to increase pressure applied to the nip portion, deviation force is increased due to a difference in parallelism between the pressure roller and the fixing film or unevenness in the thickness of an elastic layer provided on the pressure roller.
When the life span of the heating and fixing apparatus is increased, sliding friction continuously occurs between a protective sliding layer of the heater and the inner circumferential surface of the base layer of the fixing film for a long time. As a result, the surface properties of the protective sliding layer and the inner circumferential surface of the base layer deteriorate, and the friction therebetween is increased. When the friction is increased, the deviation force applied to the fixing film is increased.
As such, with an increase in the process speed and the life span of the heating and fixing apparatus, deviation force applied to the end surface of the base layer of the fixing film is increased. However, the shape of the end surface of the base layer is not considered. The fixing film deviates in the longitudinal direction by the deviation force, but the deviation of the fixing film is regulated by the end regulating surface of the fixing flange. When the deviation force is increased, strong stress is applied to the end surface of the base layer.
When the entire end surface of the base layer of the fixing film is rounded, the contact area between the end surface of the base layer and the end regulating surface of the fixing flange is reduced.
FIG. 10 is a diagram illustrating the contact between a base layer end surface 71a of a fixing film 71 and an end regulating surface 76 of a fixing flange 75 when the base layer end surface is rounded.
When the fixing film is operated for a long time in the state shown in FIG. 10, the base layer end surface 71a does not endure strong deviation force since the base layer end surface 71a of the fixing film 71 comes into point contact with the regulating surface. As a result, the fixing film 71 is likely to be damaged.
When the edge of the end surface of the base layer of the surface fixing film is not rounded, the contact area between the end surface of the base layer of the fixing film and the end regulating surface of the fixing flange is large.
FIG. 11A is a diagram illustrating the contact between the base layer end surface 71a and the end regulating surface 76 of the fixing flange 75 when an edge 71a1 of the base layer end surface 71a of the fixing film 71 is not rounded. FIG. 11B is a diagram illustrating the contact between a portion of the edge 71a1 of the base layer end surface 71a of the fixing film 71 shown in FIG. 11A and the end regulating surface 76 of the fixing flange 75.
As shown in FIG. 11A, when the edge 71a1 of the base layer end surface 71a of the fixing film 71 is not rounded, the contact area between the base layer end surface 71a of the fixing film 71 and the end regulating surface 76 of the fixing flange is large even when a strong deviation force is applied to the fixing film 71. Therefore, the end regulating surface 76 can disperse the deviation force of the fixing film 71. However, as shown in FIG. 11B, when the fixing film 71 is not parallel to a pressure roller (not shown), a portion of the edge 71a1 of the base layer end surface 71a contacts with the end regulating surface 76 of the fixing flange 75. Then, the deviation force is concentrated on one point of the edge 71a1 contacted with the end regulating surface 76. Therefore, the fixing film 71 is likely to be damaged.
As described above, the shape of the end surface of the base layer of the fixing film is very important in order to prevent the damage of a film.